Re: Performance issue of 'git branch'

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Jul 23, 2009, 03:08 UTC
Message-ID: <alpine.LFD.2.01.0907221959330.21520@localhost.localdomain>
In-Reply-To: <alpine.LFD.2.01.0907221921570.3352@localhost.localdomain>

On Wed, 22 Jul 2009, Linus Torvalds wrote:

Show 7 quoted lines

> 
> It uses the "raw" version of 'for_each_ref()' (which doesn't verify that 
> the ref is valid), and then does the "type verification" before it starts 
> doing any gentle commit lookup.
> 
> That should hopefully mean that it no longer does tons of object lookups 
> on refs that it's not actually interested in.

Hmm. On my kernel repo, doing

	GIT_DEBUG_LOOKUP=1 git branch | wc -l

I get
 - before: 2121
 - after: 39

(where two of the lines are the actual 'git branch' output). So yeah, this should make a big difference. It now looks up just two objects (one of them duplicated because it checks "HEAD" - but the duplicate lookup won't result in any extra IO, so it's only two _uncached_ accesses).

The GIT_DEBUG_LOOKUP debug output probably does match the number of cold-cache IO's fairly well for something like this (at least to a first approximation), so I really hope my patch will fix your problem.

			Linus